The invention relates to a cryogenic pump for pumping cryogenic fluids, such as liquified oxygen etc., but particularly cryogenic hydrocarbons used in hydrocarbon fuel dispensing operations.
Compressed and liquified hydrocarbon gases, typically natural gas which is mostly methane (CH.sub.4), have been used for powering vehicles for some time. Compressed natural gas (CNG) is commonly stored at ambient temperatures at pressures of between 2,400 and 3,000 PSI(16,637 and 20,771 kPa), and is unsuitable for trucks and buses due to the limited operating range and heavy weight of the CNG storage tanks.
On the other hand, liquified natural gas (LNG) is normally stored at temperatures of between about -240.degree. F. and -200.degree. F., (about -150.degree. C. and -130.degree. C.) and at pressures of between about 15 and 100 PSIG (204 and 790 kPa) in a cryogenic tank, providing a power density of about four times that of CNG. While LNG has a greater potential for use with buses and trucks than CNG due to this higher power density, problems still exist with both the pumps used at the fuelling stations, and vehicle tanks mounted within the vehicles. For example, at prior art fuelling stations, venting of LNG during the fuelling process in the order of 10% of total fuel delivered is common, and this loss can be attributed to problems associated with the fuel dispensing pump and pressure differential between the storage tank and the vehicle tank. In addition, due to difficulties in determining accurately the actual amount of fuel in the vehicle tank during filling, vehicle tanks are either unintentionally overfilled, risking tank rupture or compounding venting losses, or alternatively, due to the desire of the fuelling operator to reduce venting losses, vehicle tanks are unintentionally only partially filled, and consequently vehicles often exhaust their fuel supply within a short period of having been refuelled.
With respect to the delivery pumps used at fuelling stations, most prior art pumps have a relatively low pump delivery pressure which presents problems as follows. In order to reduce fuelling time to a few minutes, minimum fuelling rates of 25 gallons (100 liters) per minute are desirable, which requires a relatively high pressure drop in the order of 30 to 60 PSIG (310 to 516 kPa) between the pump discharge and the vehicle tank. To sustain flow, the vehicle tank must be vented during the fuelling operation with substantial venting losses. In addition, at the start of each fuelling operation, the valves and relatively large fuelling hoses etc. must be cooled which further increases venting losses. Venting of gases from the vehicle tank requires that the fuelling hose contains both a filling line and a venting line, or two separate hoses are required. Because these hoses contain cold fluid they must be insulated, and the disconnect nozzles are extremely bulky and awkward to handle.
To the inventor's knowledge, prior art cryogenic pumps used for this type of service are centrifugal pumps, which are placed either in the liquid inside the storage tank, or below the storage tank in a separate chamber with a large suction line leading from the tank, with both the pump and suction line being well insulated. Because a cryogenic liquid is always at its boiling temperature when stored, any heat leaked into the suction line and any reduction in pressure will cause vapour to be formed. Thus, if the centrifugal pump is placed outside the tank, vapour is formed and the vapour will cause the pump to cavitate and the flow to stop. Consequently, all prior art cryogenic pumps known to the present inventor require a positive feed pressure to prevent or reduce any tendency to cavitation of the pump. The positive feed pressure is attained by locating the pump several feet, e.g. 5-10 feet (about 2-3 meters) below the lowest level of the liquid within the tank, and such installations are usually very costly. Also, centrifugal pumps cannot easily generate high discharge pressures which are considered necessary to reduce fuelling time.
Reciprocating piston pumps have been used for pumping LNG when high discharge pressures are required, but such pumps also require a positive feed pressure to reduce efficiency losses that can arise with a relatively high speed piston pump. Prior art LNG piston pumps are crankshaft driven at between 200 and 500 RPM with relatively small displacements of approximately 10 cubic inches (164 cu. cms). Such pumps are commonly used for developing high pressures required for filling CNG cylinders and usually have a relatively low delivery capacity of up to about 5 gallons per minute (20 liters per minute). Such pumps are single acting, i.e. they have a single chamber in which an induction stroke is followed by a discharge stroke, and thus the inlet flow will be stopped half of the time while the piston executes the discharge stroke. Furthermore, as the piston is driven by a crank shaft which produces quasi-simple harmonic motion, the piston has a velocity which changes constantly throughout its stroke, with 70% of the displacement of the piston taking place during the time of one-half of the cycle, i.e. one-half of the stroke, and 30% of the piston displacement occurring in the remaining half cycle time. The variations in speed of the piston are repeated 200-500 times per minute, and generate corresponding pressure pulses in the inlet conduit, which cause the liquid to vaporize and condense rapidly. This results in zero inlet flow unless gravity or an inlet pressure above boiling pressure of the liquid forces the liquid into the pump. In addition, the relatively small displacement of these pumps results in relatively small inlet valves which, when opened, tend to unduly restrict flow through the valves. Thus, such pumps require a positive inlet or feed pressure of about 5 to 10 PSIG (135-170 kPa) at the feed or inlet of the reciprocating pump unless the inlet valve is submerged in the cryogenic liquid in which case the feed pressure can be reduced. Large cryogenic piston pumps, with a capacity of about 40 gallons per minute (150 liters per minute) have been built, but such pumps are designed for very high pressure delivery, require a positive feed pressure, and are extremely costly.
With respect to the vehicle tanks, all prior art cryogenic tanks known to the inventor can only be filled partly due to the requirement for an ullage or vapour space above the liquid, which space is dependent on pressure setting of a relief valve. A cryogenic tank is full when liquid within the tank occupies full tank volume at a temperature which has a corresponding boiling pressure equal to the pressure setting of the relief valve. Thus, the colder the liquid, the less volume it occupies and the greater the vapour space above the liquid. As the liquid temperature rises, the vapour space becomes smaller and eventually disappears as the liquid temperature approaches the boiling temperature corresponding to the relief valve pressure setting. Typically, the ullage space is between 10% and 13% of the full tank volume based on conventional relief valve settings. To determine volume of fuel in a tank when filling, normal practice is to provide a dip-tube on the vent or vapour line with the tube terminating at a central location of the tank, and at an elevation which, if the tank was horizontal and the liquid was steady, would provide the required ullage space. When a tank is being filled with high velocity liquid, the liquid in the tank is highly agitated, and thus there is no constant liquid level for measuring liquid volume within the tank. Normal practice is to open the vent line during filling and to watch the vented gas until liquid becomes visible, at which time the filling is stopped. However, liquid can also become visible at the start of the filling operation when the tank is warm and boiling of liquid within the tank creates a heavy mist of vapour space. To ensure the tank is as full as possible, the operator normally waits until an essentially complete liquid stream of LNG appears in the vent tube before stopping the flow, which results in some of the ullage space being filled with cryogenic liquid. When the liquid in the tank expands due to heat leak, pressure in the tank can rise rapidly and excessively, increasing the risk of rupture of the tank.
To reduce heat leak in a small cryogenic tank, commonly a single conduit is used both for liquid delivery into the tank, and liquid drainage or removal from the tank. This is a compromise solution which results in the tank being filled from the bottom of the tank, with little contact between cold liquid being pumped into the tank, and warm vapour above the liquid surface. On the other hand, in large stationary storage tanks, it is common to spray incoming liquid from an upper portion of the tank to increase contact between warm vapour and the incoming liquid which causes a fast reduction in pressure in the tank due to condensing of vapour. While this approach is used with large storage tanks, to the inventor's knowledge, it is not used with the smaller vehicle tanks. The liquid outlet conduit is considered to be hazardous with vehicle tanks, because if there is a breakage in an external line extending from the liquid outlet conduit, essentially all of the liquid in the tank is forced out through the break due to vapour pressure above the liquid. Only when essentially all liquid in the tank has been discharged will excess pressure in the tank be reduced. This hazard is of particular concern for small vehicle tanks in which the external line could be exposed to damage in a motor vehicle accident.
Natural gas burning engines can be classified into two broad classes, namely those having a low pressure fuel system and those having a high pressure fuel system. A low pressure fuel system is defined as a fuel system of an engine which operates on a fuel pressure which is lower than the minimum operating pressure of the tank. In this type of low pressure system, no fuel pump is required and the tank has a vapour conduit which removes vapour from the tank, and a liquid conduit which removes liquid from the tank. Each conduit is controlled by a respective valve, which in turn is controlled by at least one pressure sensor. The engine normally receives fuel through the liquid conduit, except in instances where tank pressure exceeds a maximum, in which case the vapour conduit is opened, so as to release some vapour to the engine, which reduces pressure in the tank, thus enabling continued operation on liquid from the tank. This is a simple system which ensures that tank pressure is kept low by taking fuel in the vapour phase from the tank whenever pressure in the tank is over a minimum level, for example about 60 PSIG (516 kPa).
In contrast, a high pressure fuel system requires a fuel pump which supplies fuel at a pressure of between 300 and 3,000 PSIG (2,168 and 20,771 kPa), depending on fuel system parameters. This is usually accomplished by a small displacement piston pump located inside the vehicle tank with a submerged inlet to ensure a positive feed pressure. Such installation is difficult to install and service, and makes the fuel tank and pump assembly relatively large. Because the pump can only pump liquid, all vapour generated by heat leak and working of the pump will decrease the holding time of the tank by a substantial amount, and result in high fuel loss because the vapour must be vented prior to refuelling the tank. This venting of vapour reduces effective capacity of the vehicle tanks still further, compounding the difficulty of use of LNG in a vehicle tank. To the inventor's knowledge, there is no single pump which can efficiently pump both liquid and vapour, or a mixture of both, and thus a system which can remove and burn vapour in the engine is not available for high pressure fuel systems. Also, conventional piston pumps require a positive pressure at the inlet port, which severely limits location of such pumps, and in particular such pumps cannot be used with a vehicle tank having a conventional "over the top" liquid outlet. Many problems would be solved if a vehicle fuel pump could be developed which could operate with a negative suction pressure which would permit the vehicle pump to be located outside the vehicle tank and placed wherever space is available in the vehicle.